These systems are under study with a goal of providing useful images of structures deep within the body without the use of ionizing radiation. There are at least three ways researchers have been trying to image in densely scattering media:
One way to image in a densely scattering medium is by measuring the time-of-flight of photons which travel in the medium and detecting those photons with the shortest travel time. Photons which experience multiple scattering well outside the beam path have a longer time-of-flight and can therefore be rejected. This technique has been suggested by Jarry et al. in their paper "Simulation of Laser Tomoscopy in a Heterogeneous Biological Medium", Medical & Biological Engineering & Computation, 1986, 24,407-414). This technique has been implemented by Takiguchi et al. as described in their paper "Laser Pulse Tomography Using a Streak Camera", Proceedings Image Detection and Quality, July 1986, and by S. Andersson-Engels et al. as described in their paper "Time-resolved Transillumination for Medical Diagnostics", Optics Letters, Vol. 15, No. 21, November, 1990. It requires sophisticated pulsed lasers, with pulse times in the picosecond to femtosecond range, and a very fast detection system. With time-of-flight systems, image resolution can be improved at the expense of signal strength.
A second way to image in a densely scattering medium uses coherent light illumination and optical heterodyning detection to reject scattered light. Because of the angular response of a heterodyned detector, it can be made sensitive only to light which exits the tissue normal to the detector axis. This technique will reject scattered light, but the technique suffers from very low signal strength, since the amount of coherent light in the medium falls off exponentially with the medium thickness. This technique has been demonstrated by researchers at the Thomson CGR research labs and by M. Toida et al. in the Inaba Biophoton Project, Japan. When applied to tissue imaging, the heterodyne and time-of-flight detection techniques are limited to imaging through about 2-3 cm tissue owing to the low signal levels of the system.
A third way to image in a densely scattering medium uses an optically heterodyned detector in conjunction with sound waves projected into the medium. A system of this type is described by Dolfi and Micheron of General Electric CGR SA in International Publication WO 89/00278 published on the base of the Patent Cooperation Act and entitled "IMAGING PROCESS AND SYSTEM FOR TRANSILLUMINATION WITH PHOTON FREQUENCY MARKING". Dolfi and Micheron use the fact that a sound wave projected into the medium causes the scatterers in the medium to vibrate. Light which is scattered by the medium therefore picks up a Doppler shift equal to the medium's vibration frequency. Dolfi and Micheron detect variations in the intensity of this Doppler shift by heterodyning the Doppler modulated light passing through the medium with unmodulated light, then selecting for the Doppler shift frequencies with electronic filters. Despite the directionality of the optical heterodyning procedures employed by Dolfi and Micheron, subsequent scattering in other portions of the scattering medium can undesirably interfere with direct imaging of the Doppler modulated light; Dolfi and Micheron describe ways to reduce this interference. These methods reduce the interference attributable to elastic scattering effects in the medium, but inelastic scattering effects in the medium can still introduce undesirable interference with direct imaging. Furthermore, since the number of photons which travel relatively straight after initial scattering is a negligible fraction of the total number of initially scattered photons, detection sensitivity tends to be poor if subsequently scattered photons remain undetected.
This invention concerns improving the measurement of light absorption in a localized region within a medium where multiple scattering dominates without rejecting multiply scattered light. Light imaging in such a medium is difficult because the random diffusion of photons prevents image formation using direct imaging techniques. If scattered light rejection techniques are used to suppress response to multiple scattering, tomographic reconstruction is still difficult because the percentage of photons which travel relatively straight after initial scattering is very low.